Shown here is a satellite image of an alleged nuclear enrichment site in Iran. (DigitalGlobe/globalsecurity.org)

An Iranian resistance group claimed Thursday that it has compiled evidence showing Iran has been secretly developing a nuclear enrichment site for years outside Tehran, a development one official said proves Iran has a "hidden, secret nuclear weapons program."

The Iran Policy Committee hosted a press conference in Washington, D.C., where research from the Iranian resistance, known as the Mujahedeen Khalq, was unveiled. The findings could not be independently confirmed.

The presenters cited evidence, including satellite imagery, they claimed was compiled after years of extensive research. They said that the Behjatabad-Abyek enrichment site is located near the city of Qazvin, about 75 miles outside of Tehran. The group claimed the regime has spent about $100 million to date on the underground project, which started in early 2005, and that construction is 85 percent complete.

They said it was being built inside mountains to withstand bombings.

Alireza Jafarzadeh, president of Strategic Policy Consulting, said the research shows the Iranian officials have been "lying through their teeth" in claiming to have disclosed all nuclear activity to international inspectors. He said it proves Iran is pursuing a nuclear weapons program, only more secretly than before -- and that the information has been turned over to the U.S. government.

A U.S. official, asked about the claims, said the government has known about the facility "for years" but urged caution.

"While there's still some ambiguity about its ultimate purpose -- not unusual for something that's still taking shape -- there's no reason at this point to think it's nuclear," the official said. "The Iranians put military stuff in tunnels, too. People should be cautious about reaching conclusions here."

But Soona Samsami, an Iranian activist, called the project a "major" enrichment site more important than the facility at Qom confirmed by Western officials last year.

The presenters said there is no evidence yet that centrifuges have been moved into the new building.

The opposition group was the first to reveal the existence of secret Iranian uranium enrichment facilities in Natanz and a heavy water facility in Arak back in 2002 -- the sites were later confirmed by U.S. intelligence. Though officials typically have confirmed revelations made by the resistance group, they have at times had a mixed record and are listed on the State Department's terror group list -- a list representatives argue should not include them. The European Union took them off their terror list last year.

U.S. officials could not confirm the credibility of the nuclear site claims Thursday, but State Department spokesman P.J. Crowley said the administration would "study" the information provided by the group.

"The MEK has made pronouncements about Iranian facilities in the past -- some accurate, some not," he said.

Iranian officials have defended their right to nuclear development but claim their research is for peaceful purposes only. The regime raised alarm last month when it started up its first nuclear reactor at the Russian-built Bushehr plant.